Originally Posted by bruceboyles
Thanks for the suggestion. The original hatch cover was fiberglass. I don't think canvas would work, it would be too easy to step through.
Sorry I thought you needed a hatch cover, you need a hatch.
Paul323 has the idea. Possibly make a hatch mould to fit your opening using door skin or ply, fillet the joints and smooth the inside with plaster, fairing compound or anything else that can be worked and sanded to a perfect finish. Apply wax or release agent and lay up the hatch as you like.